1. Field of the Invention
The present invention relates to a storage type water heater. In particular, the present invention relates to a storage type water heater capable of detecting an abnormality of a temperature sensor disposed on a circulation pipe line.
2. Description of the Related Art
There has been known a conventional storage type water heater comprising: a stored hot water tank for storing stored hot water; a heating unit including a heat exchanger for heating the stored hot water, a gas burner for heating the heat exchanger, and an air supply fan for supplying combustion air to the gas burner; a circulation pipe line for circulating the stored hot water between the stored hot water tank and the heating unit; and a circulating pump for circulating the stored hot water in the circulation pipe line. In this kind of the storage type water heater, in order to heat the stored hot water, the stored hot water is supplied from the stored hot water tank toward the heating unit by actuating the circulating pump, combustion air is blown to the gas burner by actuating the air supply fan, and the gas burner is burnt, whereby heat exchanging in a heat exchanger is operated and the stored hot water heated up to a set temperature is returned to the stored hot water tank. Further, in order to return the stored hot water, which has been heated up to the set temperature, to the stored hot water tank, a temperature sensor for adjusting temperature of the stored hot water is disposed on an outlet side pipe interposed between the stored hot water tank and the heating unit. Specifically, the temperature of the stored hot water detected by the temperature sensor for adjusting the temperature is compared with the set temperature, whereby a flow rate of the stored hot water flowing in the circulation pipe line and a supply amount of gas to be supplied to the gas burner are controlled.
However, in the case where malfunction occurs in the temperature sensor for adjusting temperature or a difference between the temperature detected by the temperature sensor for adjusting the temperature and an actual temperature of the stored hot water is caused by a secular change, there arises a problem that combustion is controlled based on a temperature other than an actual temperature. In view of this, it is desirable to determine an abnormality of the temperature sensor; however, if one temperature having a single thermistor is only used, it is difficult to determine the abnormality of the temperature sensor due to the secular change. A temperature sensor incorporating two thermistors in one protective cylindrical case has been proposed as a temperature sensor for use in a water heater. (For Example, Japanese Unexamined Patent Publication No. S60-73324) According to the temperature sensor including the two thermistors therein, it is possible to determine the abnormality of the temperature sensor by detecting a temperature difference between both of the thermistors. However, such a temperature sensor is expensive, thereby inducing a disadvantage from the viewpoint of a cost. Otherwise, it is conceived that temperature sensors are disposed on not only an outlet side pipe but also an inlet side pipe between a stored hot water tank and a heating unit. According to the storage type water heater having the two temperature sensors, since both of the temperature sensors can detect inlet and outlet side temperatures of the stored hot water, respectively, it is possible to calculate a difference between temperatures of the two temperature sensors by comparing the detected temperatures even when the storage type water heater is operated in a heat retaining operation, whereby the abnormality of the temperature sensor can be determined. However, in the case where the storage type water heater is operated in the heat retaining operation, the abnormality of the temperature sensor needs to be predictively determined based on various factors such as the inlet side temperature of the stored hot water supplied from the stored hot water tank, the outlet side temperature of the stored hot water returned from the heating unit to the stored hot water tank, a heating amount of a gas burner, and a flow rate of the stored hot water flowing in the circulation pipe line. Accordingly, there is a problem that a complicated predictive calculation control is required.